Multi-parameter distributed fiber optic sensing using double-Brillouin peak fiber in Brillouin optical time domain analysis.

In this paper, we demonstrate a multi-parameter fiber sensing system based on stimulated Brillouin scattering in a double-Brillouin peak specialty fiber with enhanced Brillouin gain response. The amplitude level of the second Brillouin gain peak, which originated from the higher-order acoustic modes, has been improved with an approximately similar amplitude level to the first Brillouin gain peak from the fundamental acoustic mode. Compared to other multi-Brillouin peak fibers presented in the literature, the proposed fiber significantly reduces the measured Brillouin frequency shift error, thus improving strain and temperature accuracies. By utilizing the sensitivity values of the strain and temperature associated with each Brillouin gain spectrum (BGS) peak, a successful discriminative measurement of strain and temperature is performed with an accuracy of ±13 µÉ›, and ±0.5 °C, respectively. The proposed double-Brillouin peak fiber appears to be a possible alternative to other multi-BGS peak fibers, for instance, large effective area fiber and dispersion compensating fibers, which are inherently accompanied by large measurement errors due to the weak Brillouin gain values originating from the higher-order acoustic modes. The demonstrated results show different strain and temperature coefficients of 47 kHz/µÉ›, 1.15 MHz/°C for peak 1 and 51 kHz/µÉ›, 1.37 MHz/°C for peak 2. Moreover, the enhanced BGS peak gains having nearly the same amplitude levels enable the discriminative measurement of strain and temperature. Such fibers in Brillouin interrogation eliminate the need for complex monitoring setups and reduce measurement errors. We recommend that for long-distance natural gas pipeline monitoring, where discriminative strain and temperature measurement is crucial, the proposed double-Brillouin peak fiber can be highly beneficial.

Robust Vector BOTDA Signal Processing with Probabilistic Machine Learning.

This paper presents a novel probabilistic machine learning (PML) framework to estimate the Brillouin frequency shift (BFS) from both Brillouin gain and phase spectra of a vector Brillouin optical time-domain analysis (VBOTDA). The PML framework is used to predict the Brillouin frequency shift (BFS) along the fiber and to assess its predictive uncertainty. We compare the predictions obtained from the proposed PML model with a conventional curve fitting method and evaluate the BFS uncertainty and data processing time for both methods. The proposed method is demonstrated using two BOTDA systems: (i) a BOTDA system with a 10 km sensing fiber and (ii) a vector BOTDA with a 25 km sensing fiber. The PML framework provides a pathway to enhance the VBOTDA system performance.

Pipeline Monitoring Using Highly Sensitive Vibration Sensor Based on Fiber Ring Cavity Laser.

A vibration fiber sensor based on a fiber ring cavity laser and an interferometer based single-mode-multimode-single-mode (SMS) fiber structure is proposed and experimentally demonstrated. The SMS fiber sensor is positioned within the laser cavity, where the ring laser lasing wavelength can be swept to an optimized wavelength using a simple fiber loop design. To obtain a better signal-to-noise ratio, the ring laser lasing wavelength is tuned to the maximum gain region biasing point of the SMS transmission spectrum. A wide range of vibration frequencies from 10 Hz to 400 kHz are experimentally demonstrated. In addition, the proposed highly sensitive vibration sensor system was deployed in a field-test scenario for pipeline acoustic emission monitoring. An SMS fiber sensor is mounted on an 18" diameter pipeline, and vibrations were induced at different locations using a piezoelectric transducer. The proposed method was shown to be capable of real-time pipeline vibration monitoring.

Multiplexable high-temperature stable and low-loss intrinsic Fabry-Perot in-fiber sensors through nanograting engineering.

This paper presents a method of using femtosecond laser inscribed nanograting as low-loss- and high-temperature-stable in-fiber reflectors. By introducing a pair of nanograting inside the core of a single-mode optical fiber, an intrinsic Fabry-Perot interferometer can be created for high-temperature sensing applications. The morphology of the nanograting inscribed in fiber cores was engineered by tuning the fabrication conditions to achieve a high fringe visibility of 0.49 and low insertion loss of 0.002 dB per sensor. Using a white light interferometry demodulation algorithm, we demonstrate the temperature sensitivity, cross-talk, and spatial multiplexability of sensor arrays. Both the sensor performance and stability were studied from room temperature to 1000°C with cyclic heating and cooling. Our results demonstrate a femtosecond direct laser writing technique capable of producing highly multiplexable in-fiber intrinsic Fabry-Perot interferometer sensor devices with high fringe contrast, high sensitivity, and low-loss for application in harsh environmental conditions.

Raman scattering in single-crystal sapphire at elevated temperatures.

Sapphire is a widely used high-temperature material, and this work presents thorough characterization of all the measurable Raman scattering modes in sapphire and their temperature dependencies. Here, Raman scattering in bulk sapphire rods is measured from room temperature to 1081°C and is illustrated as a method of noncontact temperature measurement. A single-line argon ion laser at 488 nm was used to excite the sapphire rods inside a cylindrical furnace. All the anti-Stokes peaks (or lines) were observable through the entire temperature range of interest, while Stokes peaks were observable until they were obscured by background thermal emission. Temperature measurements were found to be most reliable for A1g and Eg modes using the peaks at ±418, ±379, +578, +645, and +750 cm-1 (+ and - are designated for Stokes and anti-Stokes peaks, respectively). The 418 cm-1 peak was found to be the most intense peak. The temperature dependence of peak position, peak width, and peak area of the ±418 and ±379 peaks is presented. For +578, +645, and +750, the temperature dependence of peak position is presented. The peaks' spectral positions provide the most precise temperature information within the experimental temperature range. The resultant temperature calibration curves are given, which indicate that sapphire can be used in high-temperature Raman thermometry with an accuracy of about 1.38°C average standard deviation over the entire >1000°C temperature range.

Pilot-scale testing of natural gas pipeline monitoring based on phase-OTDR and enhanced scatter optical fiber cable.

In this paper, we present the results of lab and pilot-scale testing of a continuously enhanced backscattering, or Rayleigh enhanced fiber cable that can improve distributed acoustic sensing performance. In addition, the Rayleigh-enhanced fiber is embedded within a tight buffered cable configuration to withstand and be compatible for field applications. The sensing fiber cable exhibits a Rayleigh enhancement of 13 dB compared to standard silica single-mode fiber while maintaining low attenuation of ≤ 0.4 dB/km. We built a phase-sensitive optical time domain reflectometry system to interrogate the enhanced backscattering fiber cable both in lab and pilot-scale tests. In the laboratory experiment, we analyzed the vibration performance of the enhanced backscattering fiber cable and compared it with the standard single-mode telecom fiber. Afterward, we field validated for natural gas pipeline vibration monitoring using a 4-inch diameter steel pipeline operating at a fixed pressure level of 1000 psi, and a flow rate of 5, 10, 15, and 20 ft/s. The feasibility of gas pipeline monitoring with the proposed enhanced backscattering fiber cable shows a substantial increase in vibration sensing performance. The pilot-scale testing results demonstrated in this paper enable pipeline operators to perform accurate flow monitoring, leak detection, third-party intrusion detection, and continuous pipeline ground movement.

Angular output of hollow, metal-lined, waveguide Raman sensors.

Hollow, metal-lined waveguides used as gas sensors based on spontaneous Raman scattering are capable of large angular collection. The collection of light from a large solid angle implies the collection of a large number of waveguide modes. An accurate estimation of the propagation losses for these modes is required to predict the total collected Raman power. We report a theory/experimental comparison of the Raman power collected as a function of the solid angle and waveguide length. New theoretical observations are compared with previous theory appropriate only for low-order modes. A cutback experiment is demonstrated to verify the validity of either theory. The angular distribution of Raman light is measured using aluminum and silver-lined waveguides of varying lengths.

Conversion of a TEM10 beam into two nearly Gaussian beams.

To achieve high output power and/or high efficiencies, laser systems are often designed to operate with either high-order or multiorder transverse modes. This comes at the expense of beam quality. Herein, a technique for converting the two lobes of a higher-order (Hermite-Gaussian TEM(10)) beam into two TEM(00) beams is discussed. It is shown that nearly 94% of the power in a single lobe of the TEM(10) beam spatially overlaps an appropriately chosen TEM(00) beam. Beam quality (defined by the M(2) beam quality factor) is increased by separating the two TEM(10) lobes with an edge aperture. A single-mode optical fiber can be used as a spatial filter to eliminate residual higher-order mode content. This study suggests that more than 90% of the power in a TEM(10) beam can be converted into two separate TEM(00) beams.

Improved sensitivity gas detection by spontaneous Raman scattering.

Accurate, real-time measurement of the dilute constituents of a gaseous mixture poses a significant challenge usually relegated to mass spectrometry. Here, spontaneous Raman backscattering is used to detect low pressure molecular gases. Rapid detection of gases in the approximately 100 parts in 10(6) (ppm) range is described. Improved sensitivity is brought about by use of a hollow-core, photonic bandgap fiber gas cell in the backscattering configuration to increase collection efficiency and an image-plane aperture to greatly reduce silica-Raman background noise. Spatial and spectral properties of the silica noise were examined with a two-dimensional CCD detector array.

Enhanced spontaneous Raman scattering and gas composition analysis using a photonic crystal fiber.

Spontaneous gas-phase Raman scattering using a hollow-core photonic bandgap fiber (HC-PBF) for both the gas cell and the Stokes light collector is reported. It was predicted that the HC-PBF configuration would yield several hundred times signal enhancement in Stokes power over a traditional free-space configuration because of increased interaction lengths and large collection angles. Predictions were verified by using nitrogen Stokes signals. The utility of this system was demonstrated by measuring the Raman signals as functions of concentration for major species in natural gas. This allowed photomultiplier-based measurements of natural gas species in relatively short integration times, measurements that were previously difficult with other systems.

Plasmonic nanocomposite thin film enabled fiber optic sensors for simultaneous gas and temperature sensing at extreme temperatures.

Embedded sensors capable of operation in extreme environments including high temperatures, high pressures, and highly reducing, oxidizing and/or corrosive environments can make a significant impact on enhanced efficiencies and reduced greenhouse gas emissions of current and future fossil-based power generation systems. Relevant technologies can also be leveraged in a wide range of other applications with similar needs including nuclear power generation, industrial process monitoring and control, and aviation/aerospace. Here we describe a novel approach to embedded sensing under extreme temperature conditions by integration of Au-nanoparticle based plasmonic nanocomposite thin films with optical fibers in an evanescent wave absorption spectroscopy configuration. Such sensors can potentially enable simultaneous temperature and gas sensing at temperatures approaching 900-1000 °C in a manner compatible with embedded and distributed sensing approaches. The approach is demonstrated using the Au/SiO2 system deposited on silica-based optical fibers. Stability of optical fibers under relevant high temperature conditions and interactions with changing ambient gas atmospheres is an area requiring additional investigation and development but the simplicity of the sensor design makes it potentially cost-effective and may offer a potential for widespread deployment.